Laser welding processed

ABSTRACT

In a laser welding method, occurrence of weld defects is effectively prevented, and a high-quality weld joint is provided; and in addition, a wide range of material processes including a laser-using de p-penetration welding technique can be implemented.  
     In keyhole welding using a laser performing output pulse modulation, the welding is conducted in accordance with a frequency conforming to a natural fr queucy of a metal molten pool.

TECHNICAL FIELD

[0001] The present invention relates to a laser welding method using alaser processor; more specifically, the invention relates to a new laserwelding method capable of efficiently preventing weld defects in laserwelding by introducing periodic waveform-controlled output variations toa pulse output to thereby activate a periodic flow of molten metal.

BACKGROUND ART

[0002] In recent years, the development has rapidly progressed in thetechnology for increasing the output capacity of a laser oscillator, andthe technology is expected to be applied to deep-penetration/high-speedwelding.

[0003] In this environment, the inventor has conducted studies andresearches regarding low-heat-input/deep-penetration welding using ahigh-output-laser as an object of building a strong and tough structure.

[0004] The most significant problems arising when achieving theaforementioned object is that the difficulty increases in maintaining akeyhole stable b cause of an increas in the large p netr tion depth, andthe possibility of causing defects such as porosities, blowholes, andcracks is thereby increased.

[0005] As a result, the inventor obtained knowledges that phenomenaoccurring at the keyhole and around the keyhole need to be appropriatelycontrolled to prevent the aforementioned defects.

[0006] To find a means for implementing the above, the inventorconducted studies and researches regarding influences of a pulsewaveform and a ratio between a peak output and a base output on theprevention of the defects. Then, the inventor found that when the flowof molten metal is controlled by introducing modulation to a laseroutput in accordance with an appropriate frequency and a waveform, theweld defects can significantly be reduced.

[0007] For example, as a pulsed-arc welding method employing control ofthe waveform of output variations, there is a method intended to preventdefects such as blowholes, cracks, and spatter by using a YAG laser.However, since a molten-pool natural oscillation frequency in not usedfor the frequency of the output variations, the frequency of the outputvariations does not activate the periodic flow of molten metal and isnot sufficient to prevent defects in the de p-p n tration welding.

[0008] There is another method int nded to pr v nt porosities that canoccur in a zinc-plated steel shoot. However, also in this case, defectsoccur according to a completely different occurrence mechanism ofdefects that are caused due to a high vapor pressure of zinc, and theobject material and the penetration depth are peculiar.

[0009] Accordingly, an object of the present invention in to provide anew laser welding method according to knowledges obtained by theinventor, with which the problems occurring with the conventionaltechniques are solved, occurrence of melt defects is prevented, ahigh-quality weld can thereby be provided, and the laser-usingdeep-penetration welding technique can be used in a wide range ofmaterial processes.

DISCLOSURE OF INVENTION

[0010] To solve the problems described above, first, the presentinvention provides a laser welding method characterized in that, whileperforming keyhole welding by a laser whose output is pulse-modulated,the output of the laser is periodically varied with a frequency whichconforms to a natural oscillation frequency of a metal molten pool.

[0011] Second, the present invention also provides a laser weldingmethod as described above, characterized in that the welding isconducted by a CO₂ laser.

[0012] Third, the present invention provides a laser welding methodcharacterized in that, while performing keyhole welding by a laser, apulse output of the laser In periodically varied with a frequencyconforming to a natural oscillation of a molten pool and a risevariation of a laser output is sloped. Fourth, the present inventionprovides a laser welding method characterized in that a fall variationof a laser output is sloped. Fifth, the present invention provides alaser welding method characterized in that both of the rise variationand the fall variation of the laser output is sloped.

[0013] Sixth, the present invention further provides a laser weldingmethod, characterized in that one of the methods described above isemployed, and a ratio (WB/WP) of a base output (WB) and a peak output(WP) is set to 0.6 or lower.

[0014] According to the present invention an described above, in thekeyhole welding using a laser processor, weld defects such asporosities, blowholes and cracks can be efficiently prevented byintroducing laser output variations with a frequency which conforms to anatural oscillation of a molten pool and a controlled waveform tothereby activate the p riodical flow of molten metal.

BRIEF DESCRIPTION OF DRAWINGS

[0015]FIG. 1 is an explanatory overview of a keyhole laser welding.

[0016]FIG. 2 in a view showing an example output waveform used in anembodiment. In the figure, WP represents a peak output, and WErepresents a base output.

[0017]FIG. 3 in a schematic view showing the behavior of a molten pool.

[0018]FIG. 4 is a view showing an example output waveform used toprevent defects.

[0019]FIG. 5 shows the relationship between the frequency and theporosity occurrence ratio, and indicates that the porosity occurrenceratio is lowest at a resonant frequency. In the figure, Pr representsthe defect occurrence ratio (ratio (%) of the sum of detected defectareas to the area of weld metal).

[0020]FIG. 6 is a view showing influences of a pulse-output rise time tuon the defect occurrence ratio Pr under the condition that a fall timeof the pulse output is 0.

[0021]FIG. 7 is a view showing influences of a fall time td of a pulseoutput on the defect occurrence ratio Pr under the condition that thepulse-output rise time tu is constant at 10 ms.

[0022]FIG. 8 show comparison views of x-ray radiographies of defectoccurrence states, wherein (a) shows an x-ray radiography in CW(continuous w ve) welding, and (b) shows an x-ray radiography in anoptimal condition according to the embodiment.

[0023] Reference symbols it the drawings represent the following: tu:rise time tD: peak output time td: fall time tb: base output time WP:peak output WD: base output Pr: defect occurrence ratio

BEAT MODE FOR CARRYING OUT THE INVENTION

[0024] The present invention has features as described above; andhereinbelow, embodiments thereof will be described.

[0025] First, the present invention is characterized in that, in keyholewelding, weld defects such as porosities, blowholes, and cracks areefficiently prevented by modulating a laser output in accordance with afrequency that conforms to molten-pool natural oscillations.

[0026] Roughly speaking, for example, as shown in FIG. 1, when weldingis performed using a laser, as th pen tration depth increases, thedifficulty increases in stably forming a keyhole, thereby increasing theprobability in occurrence of wold defects such an porosities (pores). Asexemplified in FIG. 2 as well as FIG. 3, in the above-described keyholewelding, when welding is performed using a pulse-modulated laser beamhaving an output shown in FIG. 2, a large amount of molten metalsplashes out from the inside of the keyhole (FIG. 3(b)) with intenseplasma generation when a pulse rises (t1 in FIG. 2 and FIG. 3(a)). Thesplashed out molten metal moves as wave motion on the surface of themolten pool toward the rear end (FIG. 3(c)), r flects therefrom (FIG.3(d)), and returns to the k ybole (FIGS. 3(e) and 3(f)). In connectionwith the movement of the molten metal, according to the presentinvention, the varying frequency of the output is controlled to conformto the frequency of the periodic motion of the molten pool, i.e., thenatural oscillation frequency of the molten metal. According to thisarrangement, the flow of the molton metal can be activated by resonanceof the m lton pool, and the above-described weld defects can beprevented thereby. When periodic output oscillations are generated inaccordance with a r sonant frequency of the molten metal, they can beimplement d not only with th rectangular pulse but also with any otherwaveforms.

[0027] As is apparent from the above description, in the presentinvention, the “natural frequency of the metal molten pool” refers tothe frequency of the reciprocation motion of the wave occurring on thesurface of the molten metal. As such, according to the presentinvention, in the laser welding with the output-pulse-modulated laserbeam, the pulse laser utput is controlled to conform to the frequency ofthe reciprocation motion and is thereby periodically varied.

[0028] Conventionally, there are known methods such as those representedby YAG laser welding intended to prevent weld defects such asporosities, cracks, and spatter in accordance with laser outputvariations. However, there are no other known methods than the presentinvention that has been developed with attention paid to the naturaloscillations of the metal molten pool, in which the output variations Inthe frequency that conforms to the frequency of the natural oscillationsare used to prevent weld defect.

[0029] There is another known method intended to prevent porosities thatcan occur in a zinc-plated steel sheet. However, also in this case,defects occur according to a completely different occurrence m chanismof d fects that are caused due to a high vapor pressure of zinc from thobject defect ccurrence mechanism of the present invention. Accordingly,the prevention technique in this method in essentially different.

[0030] As a matter of course, the present invention do a notspecifically limits the types of welding-target materials and weldingmaterially that is, the pr sent invention is important as a basictechnique of the laser-using keyhole welding. In addition, also thelaser oscillator may be of an arbitrarily type as long an the outputthereof can be pulse-modulated.

[0031] In particular, with a CO₂ laser being used for the high-outputlaser, the present invention is suitably applied to high-qualitydeep-penetration welding.

[0032] In a practical welding operation, through observation of themovement of the wave occurring on the surface of the molten metal, thefrequency of the pulse modulation of the laser output may be c nformedin situ to the natural oscillation frequency of the molten metal.Alternatively, the pulse frequency of the laser output may be set to afrequency preliminarily verified for the conformity.

[0033] Next, a description will first be made regarding effects of apulse waveform, a base output (Ws), and th like on the por sity contr lunder conditions where f r exampl, a peak output (WP), a defocusdistance, shielding-gas conditions, and a welding speed are metconstant. Subsequently, a description will be made regarding, forexample, influences of a pulse rise time (tu) and a pulse peak outputtime (tp) on porosity occurrence.

[0034] Defects can be controlled by individually controlling the outputfrequency as well an a pulse rise time (tu), a peak output time (tp), apulse fall time (td), a base output time (tb), a peak output (WP), and abass output (WB) that are shown in FIG. 4, as follows:

[0035] (a) The pulse frequency of the laser output is controlled toconform to the molten-pool natural oscillation frequency. An examplepulse waveform in this ease is shown in FIG. 1. According to theconformity between the molten-pool pulse frequency and the molten-poolnatural oscillation frequency, the periodical flow of the molten metalcan be activated.

[0036] (b) A ratio (WB/WP) between the bass output (WB) and the peakoutput (WP) is reduced as much as possible to increase the amplitude ofthe molten-pool oscillation. Thereby, the periodical flow of the moltenmetal can be activated.

[0037] (c) In connection with the pulse rise time (tu), when WB/WP isreduced, the Pr bability of spatter ocurr nce at the time of an abruptoutput variation is increased.

[0038] In addition, when the rise slope is excessively reduced, sincethe periodic flow in slowed, tu needs to be reduced to a level at whichspatter does not occur.

[0039] (d) in connection with the pulse full time (td), according to thevariation from the base output (WB) to the peak output (WP), the keyholesize In abruptly reduced. As such, a slope needs to be provided to thefall to prevent occurrence of d facts attributed to abrupt flow of themolten, metal into the keyhole. The pulse fall time (td) is preferablyset to a time sufficient to prevent the aforementioned defects.

[0040] (e) In connection with the peak output time (tp), when tp isexcessively long, intermittent splashing occurs during tp (peak outputtime). Thereby, the periodic flow according to the utput variations iscaused turbulent. As such, tp is preferably set to a short time in whichintermittent splashing does not occur.

[0041] In ordinary CW (continuous wave) welding, intermittent splashingof molten metal from a keyhole in caused in a natural manner due to arandom frequency.

[0042] (f) in conn ction with the bas output time (tb), sinc the rsponse of m lten metal to output variations is not fast, when tb isexcessively short, the amplitude of the molten-metal oscillationintroduced according to output variations is reduced. As such, a timethat does not influence the amplitude needs to be get.

[0043] The present invention is configured to incorporate the above. The“slope” in the present invention represents the state of outputvariations with respect to the time in the case where the output isdirectly changed from the base output (WB) t the peak output (WP) oroppositely from the peak output (WP) to the base output (WB) or in thecase where the output is substantially linearly changed therebetween.

[0044] Hereinbelow, the invention will be described in more detail withreference to embodiments. As a matter of course, the embodimentsdescribed below do not limit the present invention.

EXAMPLES Example 1

[0045] Partial penetration welding was performed on a steel plate SM490Cwhich is for general welded structure by using a pulse-modulated CO₂laser beam. A He gas was used as a shielding gas, and side-shielding wasperform d at a flow rate f 50 L/min.

[0046] A r ctangular waveform as shown in FIG. 2 was us d for the outputwaveform; and the peak output WP was set to 20 kw, and the base outputWN was set to 12 kW. Two types of duty cycles, namely a 50% duty cycleand a 70% duty cycle, were selected; and bead-on-plate welding wasperformed at various frequencies. Weld-defeat detection was performed byx-ray examinations with radiation emitted perpendicular to a laser beamaxis and a weld line from a sideface of a weld test piece. The ratio (%)of the sum of detected defect areas to the area of molten metal isdefined as a defect occurrence ratio Pr, and defect suppression effectswere evaluated according to Pr. FIG. 5 shows the results of Pr masurements performed at various frequencies. The defect suppression wasmost effective at 16 Hz with the 50% duty cycle and at a frequency of 13Hz with the 70% duty cycle.

[0047] During the welding performed under these conditions, the inventorobserved the movement of th surface of the molten pool through a highspeed camera. As a result, the inventor observed a phenomenon in which,as shown in FIG. 3, a wave occurred when the laser output rose from thebase output WB to the peak output WP proceeded toward the rear and ofthe molten pool, reflected from the rear end, and then returned to the kyh le. The p ri dic wave motion, namely, a frequ ncy f of naturaloscillations, can be expressed by the following expression, where themolten-pool length in r presented by L, and the wave speed (weldingspeed) is represented by V:

f=V/2L  (1)

[0048] Table 1 shows the value of f obtained from L and v throughhigh-speed photography in each of the cases of the 50% and 70% dutycycles and from Expression (1). Table 1 shows that the pulse-modulationfrequency with which the defect suppression is efficiently implementedproperly conforms to the natural oscillation frequency of the moltenpool at either one of the duty cycles. Thus, this proves weld defectssuch an porosities can efficiently be prevented by periodically varyingthe laser output in accordance with the frequency that r sonates withthe natural oscillations of the molten pool. TABLE 1 Duty cycle (%) 5070 Welding speed v (mm/s) 630 630 Mollen-pool length L (mm) 19.6 23.8Natural oscillation frequency f (Hz) 16 13

Example 2

[0049] Partial penetration welding was p rformed on a steel plate SM490Cwhich is for general welded structure by using a pulse-modulated CO₂laser beam. A He gas was used as a shielding gas, and side-shielding wasperformed at a flow rate of 50 L/min.

[0050] A trapezoidal waveform as shown in FIG. 4 was used for the outputwaveform; and the peak output WP war set to 20 kW, the bass output WBwas set to 8 kw, and the rise time tu and the fall time td were varied.Bead-on-plate welding was performed with a 50% duty cycle. Thepenetration depth at this case was about 20 mm.

[0051] Weld-defect detection was performed by x-ray radiographies withradiation emitted perpendicular to a laser beam axis and a weld linefrom a sideface of a weld test piece. The ratio (%) of the sum of dtected defect areas to the area of molten metal is defined as a defectoccurrence ratio Pr, and defect suppression effects were evaluatedaccording to Pr.

[0052]FIG. 6 shows influences of the pulse rise time tu to a defectoccurrence amount under the condition in which the pulse fall time tdwas net to zero second (td=0 ms).

[0053] In a short time region for which the pulse rise time tu wasreduced, the spatter occurrence rate was very high, and occurrence ofdefects such as underfills and p r siti s was concurrently made cnspicuous.

[0054] In contrast, when the pulse rise time tu was increased,occurrence of spatter as mentioned above could be suppressed.Concurrently, formation of a normal bead was enabled, and defects weremost suppressed when tu=10 ms. Thus, it can be known that the increasein tu is one of efficient defect-suppressing factors.

[0055] However, when tu was increased in excess of tu=10 ms, theperiodical oscillation to be introduced to the molten pool was dampened,and the probability f porosity occurrence was increased again.

[0056] Pr in the drawing represents a defect occurrence ratio, which isthe ratio (%) of the sum of detected defect areas to the area of moltenmetal.

[0057] Since it was found from the above experimented example that edefects are most suppressed when tu=10 ms, the evaluation was conductedfor influences of varied pulse fall time td on the defect occurrenceratio under the condition where the pulse rise time tu is set constantat tu=10 ms.

[0058]FIG. 7 shows the results of the above.

[0059]FIG. 7 indicates that, with the pulse fall time td being set long,defects are most suppressed at a reduced Pr where td=20 ms. This isbecause when the output abruptly varies from WP to WB, molten metalaround the k yhole flows into the keyhole, causing porosities to easilyremain; henc the output is controlled to slowly vary to enableoccurrence of defects to be prevented.

[0060] In addition, the results indicate that also the increase in thepulse fall time td in a factor that enables efficient defect prevention.Similar to the rise time tu described above, an optimal value exists forthe pulse fall time td, and defects are m at suppressed when the falltime td=20 ms.

[0061] However, then the fall time td is set longer than 20 ms, thedefect suppression effects are reduced again. This is attributed to thephenomenon in which the base output time tb in reduced according to theincrease in the fall time td, the laser output rises before the keyholecompletely becomes small, and the amplitude of oscillation introduced tothe molten pool at that time is reduced.

[0062] For comparison, FIG. 8 shows the results of x-ray radiographiesperformed for defect occurrence states in cases where CW (continuouswave) welding was performed and where pulse welding employing waveformcontrol was performed under optimal conditions according to the presentinvention.

[0063] An a result of the CW (continuous wave) welding, the defectoccurrence ratio was found to be Pr=1.5% in w lding with an output of18.3 kW. Under the optimal conditions obtain d in th embodiment of thepresent invention, namely, under the conditions where WB=8 kW, WP=20 kW,tu=10 ms, and td a 20 ms, the defect occurrence ratio was found to bePr=0.1%.

[0064] As above, according to the present invention, the result of thewelding employing the laser welding method under the optimal conditionswas such that the defect occurrence ratio can be reduced to {fraction(1/15)} in comparison to the CW (continuous wave) welding; that is,occurrence of defects can be suppressed at a very high efficiency.

[0065] In the embodiment described above, while the result was such thatthe efficient defect-occurrence suppression effects can be obtained whenthe ratio (WB/WP) of the base output to the peak output was 0.4.However, similar effects were obtained also wh n (WB/WP) was 0.6. Inthis case, the rectangular pulse was most effective for the defectsuppression.

INDUSTRIAL APPLICABILITY

[0066] According to the present invention, in keyhole welding using alaser processor, weld defects such as porosities, blowholes, and crackscan be efficiently prevented in a manner that introduces laser outputvariations with a frequency conforming to natural scillations of amolten pool and a contr lied waveform to th reby activate th periodicalflow of molten metal.

[0067] In addition, the present invention can be provided an a methodthat can be applied to thick-plate welding requiring a method forpreventing weld defects that are particularly serious problems. Th reby,high quality thick-plate laser welding that has hitherto been difficultcan be implemented, and the application fields of the laser welding canbe xpected to be increased.

[0068] Furthermore, according to the present invention, high efficiencythick-plate welding in enabled, and cost reduction in a production linecan be expected.

1. A laser welding method characterized in that, while performingkeyhole welding by a laser whose output is pulse-modulated, the outputof the laser is periodically varied with a frequency which conforms to anatural oscillation frequency of a m tal molten pool.
 2. A laser weldingmethod according to claim 1, characterized in that the welding inconducted by a CO₂ laser.
 3. A laser welding method characterized inthat, while performing keyhole welding by a laser, a pulse output of thelaser is periodically varied with a frequency which conforms to anatural oscillation of a molten pool and a rise variation of a laseroutput in eloped.
 4. A laser welding method characterized in that, whileperforming keyhole welding by a laser, a pulse output of the laser isperiodically varied with a frequency which conforms to a naturaloscillation of a molten pool and a fall variation of a laser output issloped.
 5. A laser welding method according to claim 4, characterized inthat a rise variation of the laser output is also sloped.
 6. A laserwelding method according to any one of claims 3 to 5, characterized inthat a ratio (WB/WP) of a base output (WB) and a peak output (WP) is setto 0.6 or lower.